You want to get closer to this prospect/donor, so, after you get off the phone call, seal your developing relationship with a handwritten note sent to their home address. A note is just the right amount of added attention, without appearing obsequious. You might write something like “Great to catch you by phone, thanks again for being a champion for [your organization]. I look forward to keeping you informed of our news.” And, of course, make it relevant to your phone conversation.